The present invention relates to optical devices and, in particular, to optical isolators.
In fiber optical communication systems, one concern is reflection induced by feedback of a light signal. The reflection causes noise which in turn, can degrade performance of communication. Usually, optical devices, such as optical isolators or circulators, are employed to prevent reflection. An optical isolator or circulator acts as an one way valve for a light signal, allowing the transmission of forward light and especially useful in preventing backward light. An important index to evaluate the performance of such an optical device is its ability to isolate the backward light.
These nonreciprocal optical devices for providing isolation are based on Faraday's discovery in accordance with which, the plane of polarized light rotates while passing through glass that is contained in a magnetic field. The amount of rotation is dependent upon (1) the component of the magnetic field parallel to the direction of light propagation, (2) the path length in the optical material, and (3) the ability of the magneto-optic material to rotate the polarization plane as expressed by the Verdet constant.
An optical isolator in its simplest form consists of a first polarizer, a Faraday rotator contained in a magnet configured so that the lines of flux are along the axis of the rotator and thus parallel to the direction of light propagation, and a second polarizer.
The performance of the optical devices is primarily defined by backward loss (the ability to block reflected light and commonly referred to as "isolation"). There are two different designs to achieve isolation. One of them is composed of a polarizer, a rotator, and an analyzer. In such a design, whether it succeeds in suppressing back-reflection mainly relies on the extinction ratio of the polarizer and the analyzer, reflected stray light from the individual elements, and the accuracy and stability of the 45.degree. Faraday rotator. In the other design that is commonly used in current optical isolators to reach a higher isolation, the polarizers arranged before and after the Faraday rotator are two birefringent wedges of the same shape. The birefringent wedges spatially separate the ordinary and extraordinary beams from each other in the backward direction so that both miss the entrance into the input fiber pigtail.
An optical isolator 100 of prior art usually comprises a first birefringent wedge 102, an optical rotator 104 surrounded by a magnet 106, and a second birefringent wedge 108, as shown in FIG. 1. Backward light beam 110 from an optic fiber (not shown) propagates along a direction parallel to the axis 112 of optical rotator 104 before entering into the first birefringent wedge 102. The first birefringent wedge 102 separates the light beam 110 into an extraordinary ray and an ordinary ray. Also, the first birefringent wedge 102 causes the propagation direction of light beam 110 slightly refracted. Another reason that may also result in the propagation direction of light beam 110 refracted is inaccurate installation of birefringent wedge 102 during assembling. Due to the slanted surface of first birefringent wedges 102 and/or its inaccurate installation, the propagation direction of the light beam 110 is deflected from the axis 112 of optical rotator 104 in spatial space after it enters into the first birefringent wedge 102. However, a magnetic field 120 directed parallel to the axis 112 of the optical rotator 104 may not at this time parallel to direction 118 of light beam, which affects the amount of rotation that is dependent upon the component of the magnetic field parallel to the direction of light propagation. Accordingly, it is difficult for the prior art optical isolators to achieve high isolation.
U.S. Pat. No. 5,111,330 to VanDelden et al., "VanDelden herein", disclosed a method by which the position of a magnet with respect to the optical rotator is adjusted along a direction parallel to the axis of the magnet, so as to vary the magnetic field strength applied to the optical rotator.
U.S. Pat. No. 5,602,673 to Swan, "Swan herein", taught an optical isolator in which an input transmission element has a beveled surface that is parallel to the surface of the first birefringent wedge in order to force the light traveling in the first birefringent wedge along the axis of the optical rotator.
U.S. Pat. No. 5,661,829 to Zheng, "Zheng herein", referred to adjust the orientation of the first polarizer and the second polarizer so that the isolation peak position can be modified.
Those prior art optical isolators focuses on only variation of the magnetic field strength or a saturated magnetic field strength that leads a light beam within the optical rotator of the optical isolator to be rotated to a proper extent, and the adjustment is simply conducted on two-dimension plane rather than in a three-dimension space, so the requirement that the propagation direction of a light beam shall be parallel to the direction of the magnetic field is not satisfied. Also, since the polarizers are too small in size to be accurately adjusted manually, the efforts to achieve high isolation by manually adjusting the orientation of the polarizer is a random process without a definite pattern, so it closely associates with experience.
Most importantly, prior art optical isolators ignore such a point that in order to achieve a high isolation, the direction of magnetic field and light propagation should be parallel. To achieve this aim in practical operation, the amount of rotation of Faraday rotator has to be adjusted to a proper extent relative to orientation of the two polarizers. Specifically, all three conditions discovered by Faraday must be met to obtain an optimum rotation to achieve a high isolation. The requirement that light propagation should be parallel to the direction of magnetic field also determines the accuracy of optical rotators.
Currently, these optical devices are subjected to several constrains. Only fewer than 30% of the devices so constructed can achieve an isolation higher than 40 dB at a fixed single operating wavelength in high-volume mass production. Furthermore, while wavelength is adjustable in prior art optical isolator, the isolation is lower than the value commonly required with the complexity of manufacturing. Thus, prior art optical isolators render a relatively low isolation on an average at a predetermined wavelength in high-volume mass production.
Therefore, what is needed is an optical device that optimizes the isolation at various wavelengths. Specifically, both direction and strength of the magnetic field of the optical device are adjustable in three-dimension space and preferably, the direction of the magnetic field is parallel to the direction of light propagation in three-dimension space.